1. Field of Invention
The present invention relates to the field electro-optics, particularly to color-switchable, harmful-radiation-blocking glasses. Such glasses find application in the laser industry (particularly medicine) and can also be utilized as fashion eyeware that changes the spectral content of incident radiation.
2. Description of Prior Art
At the present time, in the field of electro-optics, several methods exist for modifying the spectral composition of radiant energy.
A liquid crystal welding lens assembly, described in U.S. Pat. No. 4,039,254 to T. B. Harsch, 1977, employs several nematic-phase liquid crystal light shutters and three neutral polarizers alternately arranged in tandem. A nematic phase is a phase of a liquid crystal (LC) the liquid has a single optical axis in the direction of an applied electric field. A polarizer is a device which orients an incident electromagnetic wave in a predetermined direction With no electric field applied across the respective liquid-crystal cells, about 0.01% of the incident light will pass through the sandwich structure. On the other hand, when the electric field is simultaneously applied across the liquid-crystal cells, about 9% of incident light will be transmitted. Incorporated into the lens assembly is an electronic unit having a manually-operated ON-OFF switch. The electronic control system of the welding lens assembly utilizes two phototransistors and two light filters, one of which passes visible light, other of which passes infrared energy only.
One of the disadvantages of the above lens assembly is its considerable size. This is because the device contains an infrared protective window which is very thick. Moreover, the welding lens blocks all visible light, providing extremely poor visibility in the absence of the welding arc. Furthermore, the lens can not selectively block a specific spectral range of light. Also, the specific design of the liquid crystals of the device limits the minimum achievable switching time of the lens to approximately 50 ms.
An apparatus disclosed in U.S. Pat. No. 4,416,514 to W. T. Plummer, 1983, includes neutral, cyan, magenta, and yellow polarizers, interposed in a series of twisted nematic cells. By varying the volta applied to each cell, the twist angles of the liquid-crystal molecules change, imparting a variable rotation of the light exiting the cell. The colored polarizers cooperate with this controllably twisted light to produce desired colors.
However, the above apparatus has several drawbacks. Parallax, inherent in any stacked optical system, is present because the path of light shifts slightly as radiation travels through the liquid crystal cells. Furthermore, this apparatus is not sensitive to the time dependence of incident light, i.e., it cannot automatically block out harmful radiation if its intensity achieves the threshold harmful for human vision and it will not let more light through as the radiation level decreases. That is, if the intensity of incident radiation changes as a function of time, this device is not able to synchronously control the transmissivity of incident light.
Moreover, the devices described above produce contamination of the desired color, which is due to the fact that the utilized LC cells used are not tunable to a particular desired color.
A display system, shown in U.S. Pat. No. 4,917,465 to A. R. Conner et al., 1990, is formed by stacking three supertwisted nematic (STN) LCD panels that are tuned to different subtractive primary colors, i.e., yellow, cyan, and magenta. Interposed between the panels are polarizers, some of which may be colored to enhance color characteristics and to improve brightness. An STN panel is an LC cell in which the distance between the support plates is smaller than 10 .mu.m and the total twist of the LC is between 180.degree. and 360.degree., preferably about 270.degree.. The STN cells provide a direct-multiplexed display composed of a plurality of pixels. For a high resolution color display, an arrangement of at least 640 by 640 pixels is required.
This display system requires a pixel array controlled by a sophisticated electronic driver system, which is expensive and is difficult to manufacture. Since display systems of this type are intended to create images, they require a powerful light source. Moreover, common problems associated with pixel arrays are low brightness and poor color contrast.
Besides the disadvantages mentioned above, all of the above prior-art systems utilize plastic polarizers placed at the periphery of the liquid-crystal cells, leaving them exposed to the elements. Such an arrangement allows dirt contamination and, in some cases, physical damage of the polarizers.